The heart of any ultrasound (imaging) system is the ultrasound transducer which converts electrical energy in acoustic energy and back. Traditionally these ultrasound transducers are made from piezoelectric crystals arranged in linear (1-D) transducer arrays, and operating at frequencies up to 10 MHz. However, the trend towards matrix (2-D) transducer arrays and the drive towards miniaturization to integrate ultrasound (imaging) functionality into catheters and guide wires has resulted in the development of so-called capacitive micro-machined ultrasound transducer (CMUT) cells. A CMUT cell comprises a cavity underneath the cell membrane. For receiving ultrasound waves, ultrasound waves cause the cell membrane to move or vibrate and the variation in the capacitance between the electrodes can be detected. Thereby the ultrasound waves are transformed into a corresponding electrical signal. Conversely, an electrical signal applied to the electrodes causes the cell membrane to move or vibrate and thereby transmitting ultrasound waves.
The membrane of the CMUT cell in general consists of several materials or layers, for example a metal electrode embedded in oxides or silicone nitride. Residual stress in these layers causes the cell membrane to bend upwards or downwards, depending on the sign or direction of the stress. Therefore, the cell membrane has a bow or bending of a specific amount and in a direction (upwards or downwards). This bow or bending causes a shift in electrical and acoustical properties of the cell. For example, it influences the collapse voltage and, assuming constant bias voltage, also the centre frequency. Efforts have been made to solve this problem. For example, the paper “Fabrication of CMUT Cells with Gold Center Mass for Higher Output Pressure”, Hyo-Seon Yoon et al., 10th International Symposium on Therapeutic Ultrasound (ISTU 2010) AIP Conf. Proc. 1359, 183-188 (2011) discloses a way to improve the output pressure of a single CMUT cell by a modification to the basic CMUT cell structure, namely by adding a gold mass over the center of the top CMUT plate.
Furthermore, there are generally strict specifications on such an ultrasound transducer or CMUT device. Its manufacturing involves quite complex processes. In general, first a larger wafer is manufactured which is then separated into multiple dies each comprising an array of CMUT cells. A particular challenge in this respect is the yield loss in manufacturing when trying to meet the strict specifications for the ultrasound transducers or CMUT devices.